1. Field of the Invention
The present invention relates to an improved method of, and apparatus for, making electric light sources and discharge lamp; more particularly, but not exclusively, electric incandescent lamps and tubular fluorescent lamps.
2. Description of the Prior Art
To put the invention and its advantages as relating to fluorescent lamps into proper context, it is considered helpful briefly to recount the essential structure of a conventional fluorescent lamp tube and conventional automatic machinery for manufacturing such tubes.
A fluorescent lamp is normally a relatively long tubular envelope made of glass. A phosphor coating is baked on the inner wall of the envelope. A glass mount assembly is sealed to each end of the elongated tubular envelope. The tube contains mercury vapour and an inert gas such as argon at low pressure so that on energization of the cathodes carried by the mount assemblies a low pressure mercury vapour discharge is created inside the tube to emit ultra-violet radiation which in turn excites the phosphor on the tube wall to fluoresce and to emit visible light. The electrodes are connected to an external ballasted electrical circuit via a cap fixed to each end of the lamp.
The mount assembly has a stem which includes a pinch, a conical flare the widest region of which is to be sealed to the end of the tube, and (at at least one end, but nowadays usually at both ends) a central, slender, hollow, exhaust tube communicating with the interior of the tube so that air and other undesired gases may be exhausted therefrom and selected fill gas(es) at a desired low pressure may be introduced thereinto before finally hermetically sealing the lamp tube by fusing the exhaust tube, known as tipping-off. For the electrical connection of cathode to the external electrical circuit lead-in wires pass through and are sealed in the mount. The cathode is usually surrounded by an anti-sputtering shield supported by a stay wire also sealed in the pinch. In certain more recent constructions the shield also carries a mercury dispenser which releases a predetermined amount of mercury into the "atmosphere" of the tube interior when indirect, external heating is applied to it.
A conventional automatic machine group for making fluorescent tubes may comprise two stem-making machines and mount mills for assembling together the whole mount assembly with the lead-in wires sealed in place, conveyors for passing the mount assembly to a sealing machine which also receives hollow lamp tubes from a so-called lehr where the phosphor is baked onto the inner wall of the tubes, at an elevated temperature.
Known sealing machines are rotary turrent machines or conveyor machines rotatable either about a vertical or a horizontal axis, intermittently or continuously, and having a plurality of heads for sealing a mount assembly to each end of the tube. In a vertical sealing machine this is done by holding the tube with its axis vertical, sealing a mount assembly by means of burners with upwardly directed flames to the bottom end of the tube, removing the tube from the sealing machine and re-inserting it with its other end at the bottom for the said other end to have its mount assembly sealed thereto. In a horizontal machine the tube is held horizontally and it is possible to seal the mount assemblies to the two tube ends at the same time. From the sealing machine the tubes are transferred to an exhausting machine by means of a further conveyor. There may also be a buffer conveyor between the two machines to cope with unequal rates of output of the two machines. In the exhaust machine the cathodes are activated and all undesirable gases and volatile impurities from the activation are removed from the interior of the lamp, the required amount of mercury and filling gas are introduced and the lamp is finally tipped-off. For activation and tipping-off, the lead-in wires are splayed out to be engageable by an electrical contact-making device and to be out of the way of the usual tipping-off burner. The tubes are then provided with a cap, e.g. a bipin cap, and then the tube is passed to a cap threading machine where the lead-in wires have to be bent to the required position. The caps are then baked on the tube, passed to a pin welding or soldering machine and finally to an ageing machine.
Thus it will be noted that two turrent machines with respectively different heads are employed for sealing and exhausting, and a number of loading, unloading and transfer conveyors are required. The lead-in wires have to be manipulated at least twice, namely at the stage of activation and tipping-off, and finally for cap threading.
Furthermore, during the operation on the exhaust machine the temperature of the lamp has to be relatively high to increase the molecular motion of the gases to assist in removal through the exhaust tube, to desorb gas molecules from the glass envelope or phosphors and to remove by volatilisation moisture and other condensed vapours as well as carbon dioxide which is liberated from the material of the cathodes, usually a tungsten coil coated with earth alkaline carbonates. Thus it will be observed that the whole process has a fairly high energy consumption, yet the heating cycle is rather irrational: the tubes are first heated to a high temperature when the phosphors are baked-on in the lehr, but are allowed to cool down while in the sealing machine and then have to be re-heated for exhausting and cathode activation.
Another important irrationality of present methods and apparatus is that the hot tubes are internally relatively clean and uncontaminated in the lehr but atmospheric and other impurities are allowed free ingress in the sealing machine before sealing. These impurities must then be removed with considerable difficulty in the exhausting machine.